Surface Acoustic Wave (SAW) devices are well known as useful signal processing elements in radio frequency circuits. Such devices are generally fabricated by conventional photolithographic techniques on a crystal substrate capable of supporting acoustic waves on its surface. Metal film transducers are used to transform electrical signals to acoustic waves and vice versa. A signal present at an input transducer will generate acoustic waves which will travel over the surface of the substrate and generate an electrical response in an output transducer located at some distance from the input transducer. One useful function of SAW devices is providing precisely determined delays in RF circuits. Detailed knowledge of the propagation of surface acoustic waves and precise manufacturing techniques allow very precise delay lines to be constructed with SAW devices.
However, if such delay services are subjected to temperature excursions during operation, the temperature dependent properties of the crystal structure cause variation in the phase of the response of the SAW device. Since this phenomenon seriously effects the utility of SAW devices in many applications, various attempts have been made to design temperature stable SAW devices. For example, U.S. Pat. No. 3,943,389, which is assigned to the assignee of the present invention, presents a method of physically constraining the substrate from changes in dimension. This is a multi-layer device which involves increased manufacturing costs. Another approach is discussed in "A Novel Technique for Improving the Temperature Stability of SAW/SSBW Devices", T. I. Browning and M. F. Lewis, 1978 IEEE USP (CH-1344-1), pp. 474-477. This involves the use of multiple propagation paths aligned along different directions across the substrate, carefully balanced to provide a summed response which is relatively temperature stable. Again, this is a complex system requiring exacting manufacturing techniques.
The notation for crystal cuts used herein will conform to the standard set out in "Standards on Piezoelectric Crystals 1949", Proc. IRE 24, Dec. 1949, pp. 1378-1395.